I have a problem with my Grant Vortex condensing combi 26 Oil boiler. When I run the hot water for a bath after a few minutes the water runs luke warm, turn off the hot tap and wait a few minutes and it is hot again. It always used to work ok. The other thing I have noticed is I can hear air bubbles traveling around the heat exchanger pipes and the water store. the heating is ok and quiet. Its almost as if there is not enough water in the store? The boiler is only 4 years old but has had a few problems needing the boiler to be drained and re filled a number of times. Leaking washer on heat exchanger - leaking flow switch washer - leaking auto air vents and leaking filling loop taps. I have replaced the filling loop, air vents and washer on the exchanger. When I re fill the boiler via the filling loop does this also fill the water store? Any help as to the cool water issue would be appreciated.